While signals within telecommunications and data communications networks have been traditionally exchanged by transmitting electrical signals via electrically conductive lines, an alternative medium of data exchange is the transmission of optical signals through optical fibers. Equipment for efficiently generating and transmitting the optical signals has been designed and implemented, but the design of optical switches for use in telecommunications and data communications networks is problematic.
U.S. Pat. No. 5,699,462 to Fouquet et al., which is assigned to the assignee of the present invention, describes a switching matrix that may be used for routing optical signals from any one of a number of parallel input optical fibers to any one of a number of parallel output optical fibers. A functionally related matrix of switching elements is described in U.S. Pat. No. 4,988,157 to Jackel et al. An isolated switching element 10 is shown in FIG. 1, while a 4.times.4 matrix 32 of switching elements is shown in FIG. 2. The optical switch of FIG. 1 is formed on a substrate. The substrate may be a silicon substrate, but other materials may be used. The optical switch 10 includes planar waveguides defined by a lower cladding layer 14, a core 16 and an upper cladding layer 18. The core is primarily silicon dioxide, but with other materials that affect the index of refraction of the core. The cladding layers should be formed of a material having a refractive index that is substantially different from the refractive index of the core material, so that optical signals are guided along the core material.
The core material 16 is patterned to define an input waveguide 20 and an output waveguide 26 of a first waveguide path and to define an input waveguide 24 and an output waveguide 22 of a second waveguide path. The upper cladding layer 18 is then deposited over the patterned core material. A trench 28 is etched through the core material and the two cladding layers to the substrate. The waveguides intersect the trench at an angle of incidence greater than the critical angle of total internal reflection (TIR) when the trench is filled with a vapor or gas. Thus, TIR diverts light from the input waveguide 20 to the output waveguide 22, unless an index-matching material is located within the gap between the aligned waveguides 20 and 26. The trench 28 is positioned with respect to the four waveguides such that one sidewall of the trench passes through or slightly offset from the intersection of the axes of the waveguides.
In the 4.times.4 matrix 32 of FIG. 2, any one of four input waveguides 34, 36, 38 and 40 may be optically coupled to any one of four output waveguides 42, 44, 46 and 48. The switching arrangement is referred to as "non-blocking," since any free input fiber can be connected to any free output fiber regardless of which connections have already been made through the switching arrangement. Each of the sixteen optical switches has a trench that causes TIR in the absence of an index-matching liquid at the gap between collinear waveguides, but collinear waveguides of a particular waveguide path are optically coupled when the gaps between the collinear waveguides are filled with an index-matching fluid. Trenches in which the waveguide gaps are filled with fluid are represented by fine lines that extend at an angle through the intersections of optical waveguides in the array. On the other hand, trenches in which there exist an absence of index-matching fluid at the gaps are represented by broad lines through a point of intersection.
The input waveguide 20 of FIGS. 1 and 2 is in optical communication with the output waveguide 22, as a result of reflection at the empty gap of trench 28. Since all other cross points for allowing the input waveguide 34 to communicate with the output waveguide 44 are in a transmissive state, a signal that is generated at input waveguide 34 will be received at output waveguide 44. In like manner, input waveguide 36 is optically coupled to the first output waveguide 42, the third input waveguide 38 is optically coupled to the fourth output waveguide 48, and the fourth input waveguide 40 is coupled to the third output waveguide 46.
There are a number of available techniques for changing an optical switch of the type shown in FIG. 1 from a transmissive state to a reflective state. In the above-identified patent to Jackel et al., water or a refractive index-matching liquid resides within the gap between waveguides until an electrochemically generated bubble is formed. A pair of electrodes is positioned to electrolytically convert the liquid to gaseous bubbles. A bubble at the gap between collinear waveguides creates an index mismatch and causes light to be reflected at the sidewall of a trench. The bubble can be destroyed by a second pulse having the appropriate polarity. Removing the bubble returns the switch to the transmissive state.
Japanese application No. 6-229802 of Sato et al. (Kokai No. 8-94866) describes the use of heaters to supply and remove index-matching liquid to and from a gap that is intersected by two waveguides. The flow of liquid within a slit (i.e., trench) is controlled by selectively activating heater elements. The index-matching liquid may be a low viscosity silicon oil. Approximately one-third of the volume of the slit is filled with such liquid prior to bonding a surface cap substrate to a substrate on which the waveguides are fabricated. The fixed quantity of sealed liquid is manipulated by selectively activating one of two heater elements. Activating a first heater element locates the sealed liquid at the gap between two waveguides, while activating a second heater element removes the liquid from the gap between the waveguides.
If the liquid in the Sato et al. device were to be channeled to the trenches from the edge of the switching device, the channels would require significant space on the device footprint. The waveguides, the heaters and the thin-film electrical connections to the heaters must be properly mapped and fabricated. The concern is that the additional requirement of forming the channels would cause space requirements to exceed space availability. In an exemplary application, the center-to-center distance between parallel waveguides may be 250 .mu.m in order to match the pitch of conventional optical fiber ribbon cables. Design and production of liquid-feed channels having a sufficient volume for ensuring proper operation of the switching elements would be difficult in such an application. Another concern is that if the additional channels are formed in the waveguide substrate, there may be additional optical loss in the device.
Conversely, the concern with sealing the index-matching liquid within the trench, as taught by Sato et al., is that the volume of liquid cannot be adjusted after the device is fully assembled. That is, liquid cannot be added or removed if it is determined that the sealed volume is not optimal.
What is needed is a fabrication method and a switching device that provide sufficiently large fluid fill-channels to trenches that are intersected by closely spaced waveguides, with the formation of the fluid feed-channels not impeding upon the process of fabricating the switch structure.